1. Field of the Invention
The present invention relates to a method for driving a solid-state imaging device using a CCD (Charge Coupled Device), or the like.
2. Description of the Background Art
In recent years, solid-state imaging devices are widely used in imaging devices such as video cameras and digital still cameras. With the number of pixels of a solid-state imaging device increasing, these imaging devices are required to be able to shoot and record not only high-definition still images but also high-definition motion pictures. Moreover, these imaging devices are recently provided with various modes of operation.
Modes of operation for an imaging device may include, for example, a mode for shooting and recording still images, a mode for displaying a preview image on an LCD viewfinder provided in a camera casing, and a mode for shooting a quickly-moving object or a high-luminance object.
Typically, when shooting a still image, the exposure time is controlled by means of a mechanical shutter for exposure correction. Where the exposure time is controlled by means of a mechanical shutter, smear signals in vertical shift registers can be discarded by sweeping out the smear signals after closing the mechanical shutter. Thus, it is possible to realize a solid-state imaging device where contamination with smear signal charges is prevented, whereby it is possible to record a desirable image with very little smear noise.
Where a motion picture is recorded or where a preview image is displayed on an LCD viewfinder, an image needs to be continuously output at a frame rate of 15 to 60 frames per second so that a moving object can be recorded and displayed naturally. In view of this, image devices employ a “downsampled motion picture mode” where pixels of CCD image sensors are downsampled to reduce the number of output lines, and a “pixel-mixing motion picture mode” where signal charges from each set of pixels are mixed together to thereby reduce the total number of output pixels, thus making it possible to continuously output an image at a frame rate of 15 to 60 frames per second.
In these modes, an image is continuously taken, and therefore a mechanical shutter cannot be used. In view of this, these devices employ CCD image sensors having a vertical overflow drain structure, and the exposure is controlled by sweeping away charges (pixel signals) stored in photodiodes toward the substrate with controlled timing and by controlling an optical diaphragm. The term “vertical overflow drain structure” refers to a structure where a photodiode including an N-type layer and a P-type layer is formed on a P-type well provided on an N-type substrate, thus resulting in a vertical stack of layers as follows: P-type layer/N-type layer/P-type well/N-type substrate. Such an exposure controlling structure is referred to as an “electronic shutter”.
FIG. 4A is a timing diagram showing a vertical shift register transfer pulse (hereinafter referred to as the “φV pulse”) and a substrate sweep-out pulse (hereinafter referred to as the “φSUB pulse”) used in a conventional method for driving a solid-state imaging device using an electronic shutter. The high-voltage φSUB pulse serves as the electronic shutter described above. While the high-voltage φSUB pulse is being applied, charges stored in the photodiodes are swept out toward the substrate. While the φV pulse is being applied, the vertical shift registers transfer video signals (charges) to the horizontal shift register. In the figure, the designation “1V period” means the period of the vertical synchronization signal.
With the conventional driving method, φSUB is applied over a short period once for the period of the horizontal synchronization signal (1HD period) in order to control the signal charges stored in the photodiodes. The φSUB pulse is applied over a period in which the horizontal shift register is inoperative (referred to as the “horizontal (H) blanking period”) in each 1HD period.
With the same amount of light entering the CCD image sensors, the number of sensitivity electrons increases in a pixel-mixing motion picture mode where signals of each set of photodiodes are added together as compared with a mode where a signal of each photodiode is used individually. In a pixel-mixing motion picture mode as compared with a mode where a signal of each photodiode is used individually, the number of sensitivity electrons is controlled in an attempt to provide an appropriate exposure correction, by applying the φSUB pulse to increase the electronic shutter speed, shorten the exposure time and reduce the number of sensitivity electrons.
With this method, charges stored in the photodiodes are swept away toward the substrate by applying a pulsed high voltage (the (φSUB pulse) with a narrow width to the substrate.